The Collaborative International Dictionary of English v.0.48:

Further \Fur"ther\, a. compar. [Positive wanting; superl.
   Furthest.]
   1. More remote; at a greater distance; more in advance;
      farther; as, the further end of the field. See Farther.
      [1913 Webster]

   2. Beyond; additional; as, a further reason for this opinion;
      nothing further to suggest.
      [1913 Webster]

   Note: The forms further and farther are in general not
         differentiated by writers, but further is preferred by
         many when application to quantity or degree is implied.
         [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Furthest \Fur"thest\, a.
   superl. Most remote; most in advance; farthest. See
   Further, a.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Furthest \Fur"thest\, adv.
   At the greatest distance; farthest.
   [1913 Webster]

WordNet (r) 3.0 (2006):

furthest
    adv 1: to the greatest degree or extent or most advanced stage
           (`furthest' is used more often than `farthest' in this
           abstract sense); "went the furthest of all the children
           in her education"; "furthest removed from reality"; "she
           goes farthest in helping us" [syn: furthest,
           farthest]
    2: to the greatest distance in space or time (`farthest' is used
       more often than `furthest' in this physical sense); "see who
       could jump the farthest"; "chose the farthest seat from the
       door"; "he swam the furthest" [syn: farthest, furthest]
    adj 1: (comparatives of `far') most remote in space or time or
           order; "had traveled to the farthest frontier"; "don't go
           beyond the farthermost (or furthermost) tree"; "explored
           the furthest reaches of space"; "the utmost tip of the
           peninsula" [syn: farthermost, farthest,
           furthermost, furthest, utmost, uttermost]

Moby Thesaurus II by Grady Ward, 1.0:

22 Moby Thesaurus words for "furthest":
   at the height, at the limit, extreme, far out, farthermost,
   farthest, farthest off, furthermost, greatest, most, most distant,
   outermost, outmost, radical, remotest, too much, ultimate, ultra,
   ultra-ultra, utmost, uttermost, way out